RUS:
Крашенинников В.С. Методика оценки карстоопасности на локальном участке, расположенном в САО г. Москвы // Строительство – формирование среды жизнедеятельности. Сборник трудов 17-й Межвузовской конференции молодых специалистов. МГСУ. Москва. 2014. С. 376-381.

ENG:
Krasheninnikov V.S. Methodology for Assessing Karst Hazard at a Local Site Located in the Northern Administrative District of Moscow // Construction - Formation of the Living Environment. Proceedings of the 17th Interuniversity Conference of Young Specialists. MGSU. Moscow. 2014. Pp. 376-381.

Krasheninnikov Vadim

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

Methodology for assessing karst hazard at a local site located in the Northern administrative district of Moscow

Introduction
The study area is located in the Northern Administrative District (SАО) of Moscow, near Khoroshevskoye Highway, approximately 300 meters northwest of Begovaya metro station. According to the “Karst and Karst-Suffosion Hazard Map” (Kutepov V.M. et al., 1996) [3], the site is classified as "highly hazardous" in terms of karst processes and lies on the boundary between "highly hazardous" and "hazardous" areas regarding karst-suffosion hazards. This analysis is based on the survey data from investigations conducted in 2008.

Geological Structure of the Site

Geomorphologically, the site is situated within the third (Khodynskaya) terrace of the Moscow River floodplain. Ground surface elevations within the site range from 152.50 to 153.00 meters above sea level. The geological structure includes Quaternary (Q), Jurassic (J), and Carboniferous (C) deposits (Fig. 1). The surface is uniformly covered by a layer of sandy fill (K-Q4) with a thickness of predominantly 2.0-3.0 meters, sometimes reaching 5.0-6.0 meters. Below this, at depths of 13.5-16.0 meters, lies a layer of alluvial-fluvioglacial deposits (a,f-Q3), represented by medium-grained sands with interlayers of fine-grained sands. These deposits are underlain by moraine (g-Q2) clays, with a base depth of 16.0-21.0 meters. The thickness of the impermeable layer varies from 1.5 to 4.0 meters, occasionally up to 6.5 meters. Although the moraine serves as an impermeable layer, its thickness is insufficient to provide reliable protection from karst processes. Beneath the moraine, there are fluvioglacial deposits (f-Q2) consisting of silt and fine-grained sands, and less frequently, medium-grained sands. These fluvioglacial sands rest on the eroded surface of bedrock deposits at depths of 28.0-33.0 meters. The bedrock is composed of Upper Jurassic (J3) semi-solid clays and Upper Carboniferous (C3) limestones. The Upper Jurassic clays, which serve as a regional aquitard, are discontinuous in thickness (0-2.7 meters) and lateral extent, and in some areas, they have been completely eroded. In such areas, the fluvioglacial sands are in direct contact with the fractured limestones of the Upper Carboniferous Perkhurovskaya Formation. The limestones are fractured, in places heavily fractured or broken down to the state of rubble and lime mud.

Hydrogeological Conditions

The hydrogeological conditions of the site are complex and characterized by the presence of multiple aquifers. The first Quaternary aquifer, unconfined groundwater, is encountered at depths of around 12.0 meters within the alluvial-fluvioglacial sands, above the moraine clays. The second, supra-Jurassic aquifer of confined interstratal waters, saturates the fluvioglacial sands. Due to the erosion of Upper Jurassic clays, hydrogeological windows are formed, allowing the supra-Jurassic aquifer to connect with the Upper Carboniferous aquifer, which is located in the fractured limestones of the Upper Carboniferous. The piezometric level is established at depths of 12.5-13.0 meters from the ground surface, with hydraulic pressure heights ranging from 3.0 to 8.0 meters. The moraine clays act as the upper confining layer for the combined Upper Jurassic-Upper Carboniferous aquifer, while the Upper Jurassic clays (or, where they are eroded, the clays of the Upper Carboniferous Niverovskaya Formation) serve as the lower confining layer.

The complex hydrogeological structure, which includes the presence of multiple aquifers, both confined and unconfined, as well as the absence of a continuous aquitard separating karstifiable rocks from the overlying unconsolidated deposits, allows for a preliminary assessment of the site as hazardous in terms of both karst and karst-suffosion processes.

Fig.1 Schematic Geological-Lithological Cross-Section Along Line 1-1

Analysis of Geological Structure and Properties of the Overlying Sediments

Evaluating the geological structure and properties of the overlying deposits is critical when assessing the risk for potential geological hazards in a construction zone. This involves considering various geological factors. Specifically, assessing karst hazard in conditions of covered karst is based on the presence and characteristics of the impermeable layer that separates karstifiable rocks from the overlying soils. Additionally, the frequency of karst manifestations per unit area is taken into account [4].

However, for construction projects located in karst-prone areas, it is more important to identify hazardous zones within the boundaries of the construction site itself. While this task is more complex than a general hazard assessment, such analysis is not only feasible but has been successfully conducted by specialists in the field [1,5].

The geological investigation of the project site involves detailed studies of the soil profile, groundwater conditions, and the potential for karst processes. The reliability of the impermeable layer, combined with the frequency of karst development in the surrounding area, provides insight into the potential risks of subsidence or collapse.

Fig. 2. Distribution Maps of Average Cone Penetration Resistance (q, MPa) Across the Survey Area

It is well known that characteristic changes occur in the overlying soils above karst cavities [5]. These changes manifest as a reduction in soil strength or density, and in extreme cases, they result in various deformations of the soil layers, which can extend to the surface. The changes that precede the formation of a surface sinkhole can be detected using standard engineering-geological survey methods, such as borehole drilling and soil probing (static or dynamic). Probing allows the identification of zones of soil loosening or weakening, referred to as weakened zones, while borehole analysis reveals buried landforms and larger cavities in karstified rocks.

In cases where the site exhibits simple and predictable geological conditions from a forecasting perspective, and the weakened zone is clearly delineated and traceable from the surface to the karstifiable rocks, the localization and prediction of a potential collapse or subsidence are straightforward. The site is then classified under the highest category of karst hazard. However, in situations with thin impermeable layers or poorly expressed weakened zones, relying solely on objective data might lead to an underestimation of the karst hazard level. This kind of assessment could result in undesirable consequences, which is why a thorough and sometimes unconventional analysis is necessary in such cases.

Fig. 3. Results of Point Assessment

To identify weakened zones within the overlying strata at the study site, data from static probing were used—specifically, cone penetration resistance values (q). It is important to note that the probing was conducted "to refusal," and the data obtained did not cover the entire thickness of the Quaternary deposits, but only depths ranging from 14.0 to 19.0 m. Since the q values across the entire probing thickness were not sufficiently indicative, the analysis focused on values selected from specific intervals. The selection criteria were based on lithological homogeneity and equal depths of the layers. Clayey and sandy soils were analyzed separately.

The upper boundary of the sandy soils was determined by the maximum thickness of the fill layer—5.7 m, and the lower boundary was conditionally drawn at the minimum depth of the clayey soils—12.7 m. For the clayey soils, boundary conditions were set according to the depths of the roof and bottom. Thus, three intervals were distinguished within the sandy layer: 5.7–8.0 m, 8.0–10.5 m, and 10.5–12.7 m, while in the clayey soils, one interval was distinguished: 14.7–16.0 m.

Fig. 4. Zoning Map Combined with Bedrock Roof Map

For each interval, maps were created showing the distribution of cone penetration resistance values (q) (Fig. 2). These maps clearly highlight areas with anomalously low q values. However, it is difficult to definitively identify a specific high-risk area. To localize the most hazardous zones, a scoring system was tested. Points were assigned to each probing point within each interval, based on the average q values. Obvious "anomalies" with the lowest q values received 1 point, while points with reduced values (the lowest third of the value range) received 0.5 points. The resulting data are shown in Fig. 3.

Based on these data, a combined map (Fig. 4) was created, showing areas categorized by the risk of karst formation according to the number of points assigned. The map also includes isolines representing the bedrock roof. The most favorable conditions for karst process activation, according to the data, are observed near probing point No. 6. Other points of concern include No. 3, 9, 10, 11, 12, and 13. Particular attention should be given to anomalies that are present in all layers, including the clay layer (as stress changes in the soil first affect the clay, then the overlying sands)—points No. 9 and 13.

The southeastern part of the site, near point No. 13, is characterized by minimal moraine thickness and maximum thickness of Jurassic clays. Groundwater impacts the properties of the clay, and the likelihood of suffosion processes is minimal, allowing point No. 13 to be excluded from further consideration. However, the area around point No. 9 is classified as "very hazardous."

Moreover, another area on the site should also be considered "very hazardous." Near boreholes No. 4, 5, and 6, drilling data show that the impermeable layer has been eroded, and Quaternary sands are in direct contact with the limestones (Fig. 4), posing a risk of suffosion and particle removal into the karst-forming rocks. Additionally, during the Middle Quaternary period, a karst-suffosion sinkhole already occurred in this location, as indicated by the base of the moraine clays. These factors justify assigning the highest hazard category, as it is well-known that buried karst sinkholes pose a risk of reactivation [2].

Conclusions

This site served as the first test of a karst hazard assessment methodology based on scoring relative to static probing indicators. For the full implementation of this methodology in engineering-geological investigations, probing to the entire depth of the overlying strata is required. With modern technical means, depths of 25.0-30.0 m are quite achievable.
Two areas on the site have been identified as "very hazardous" concerning karst process activity: the area near borehole No. 9 and the area near boreholes No. 4, 5, and 6.

References

Krasheninnikov V.S., Khomenko V.P. Studying the Overlying Strata as One of the Key Components of Engineering Surveys in Covered Karst Areas // MGSU Bulletin. 2011. No. 5. Pp. 113-119.
Kutepov V.M., Kozhevnikova V.N. Stability of Karst-affected Territories. M., NAUKA, 1989.
Moscow: Geology and the City / Chief editor V.I. Osipov, O.P. Medvedev. M., AO "Moscow Textbooks and Cartography," 1997.
SP 116.13330.2012. Engineering Protection of Territories, Buildings, and Structures from Hazardous Geological Processes. Basic Provisions. Updated Edition of SNIP 22-02-2003. Ministry of Regional Development of Russia. M.: NIIOSP named after N.M. Gersevanov. 2012. 60 p.
Khomenko V.P. Regularities and Forecasting of Suffosion Processes. M., GEOS, 2003.

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